As shown in FIGS. 1 and 2, a conventional mixer consists of an oval spherical body 1, a drive shaft 2, a plurality of blade seats 3, and a plurality of blades 33. The short shaft of the oval spherical body 1 has a diameter slightly greater than the diameter of an opening A1 of a container "A". Located along the longitudinal axis of the oval spherical body 1 are a bearing and a hole 11. The drive shaft 2 is fastened pivotally with the bearing via the hole 11 of the oval spherical body 1 such that the drive shaft 2 is connected with a power source "B", and that the drive shaft 2 is fastened at another end thereof with a center holes 31 of the blade seats 3. The blade seats 3 are provided symmetrically in the peripheral surfaces thereof with a plurality of grooves 32. The grooves 32 have a center line forming an angle .theta. with the center line of the blade seats 3. The blades 33 are fastened pivotally with the grooves 32 of the blade seats 3 by means of pivots 34 such that the blades 33 are capable of turning unidirectionally and in the same direction at an angle of 90 degrees. The oval spherical body 1 has a longitudinal end which is extended out of the bottom end of the drive shaft 2. Another longitudinal end of the oval spherical body 1 has an arcuate surface 12 which is located on the opening A1 of the container "A". As the drive shaft 2 is driven, the blades 33 are caused to expand by the centrifugal force such that the blades 33 are perpendicular to the center lines of the blade seats 3, so as to bring about the action of mixing the liquid held in the container "A". The arcuate surface 12 of the oval spherical body 1 is capable of making a 360-degree rotation along the opening A1 so as to enable the drive shaft 2 to move around in the container "A".
Such a prior art mixer as described above is defective in design in that the fastening of the blades 33 in the grooves 32 must be precise and that it is technically difficult to fasten pivotally the blades 33 by means of pivots 34 with precision. In addition, the moving range of the drive shaft 2 in the container "A" is limited such that the mixing effect is compromised. When the drive shaft 2 in operation is not stabilized, the expanded blades 33 are prone to hit the inner wall of the container "A". Moreover, the mixing effect of the prior art mixer is undermined when the rotational speed of the drive shaft 2 is too slow to cause the blades 33 to expand fully.